1. Technical Field of the Invention
The present invention relates to the use of a Cartesian feedback control system to achieve power amplifier linearization in transmitter systems using linear modulations, and more particularly, to the use of soft switching within a Cartesian feedback loop control system to prevent transient spikes in transmitter systems using linear modulation.
2. Description of Related Art
A Cartesian feedback control system is commonly used to achieve power amplifier linearization in transmitter systems using linear modulation schemes, such as quaduture phase shift keying (QPSK) or quaduture amplitude modulation (QAM). A typical Cartesian feedback loop includes some form of switching wherein the feedback signal for a connected loop amplifier is taken either directly from the loop amplifier output (during the open loop or inactive state) or from the coupled, demodulated power amplifier (PA) output (closed loop or active state).
The Cartesian feedback loop includes two states. Transitions between states are performed by hard switching of the feedback signal. Normally, the Cartesian feedback loop must be switched from the opened to the closed state (inactive to active state) sometime during the transmit cycle of a transmitter each time the attached loop power amplifier comes up to full power. Each time the Cartesian feedback loop is switched into the circuit, the feedback loop produces a transient spike in the feedback signal due to minute differences in amplitude and phase of the open loop and closed loop signals. The transient spike manifests itself as a glitch in the power amplifier output. The glitch may extend across a wide frequency spectrum and violate transient adjacent channel power constraints in many existing radio systems.
The problem is particularly difficult to deal with within a time division multiple access (TDMA) based architecture wherein the loop switching must be coincident with the start of each TDMA pulse. Present solutions for reducing or minimizing the effects of the transient glitch are achieved by closing/opening the loop only under low power conditions and/or temporarily disabling the antenna connection while the feedback loop is in transition. However, additional solutions for eliminating or minimizing the power glitch during feedback loop transitions without adversely affecting the operation of the transmitter are needed.